With the recent development of lighter or thinner, higher-performance versions of IC cards, electronic devices, etc., circuit patterns of conductor circuit boards incorporated therein tend to become still higher in density and less in thickness. Accordingly, there is a demand for a method for efficiently mass-producing such conductor circuit boards of high quality.
As a conventional method of producing conductor circuit boards, there is the so-called etching method in which a copper foil with a thickness of 18 .mu.m to 35 .mu.m or more is bonded to the surface of an insulating substrate, made of e.g. glass-epoxy resin, for lamination, the surface of the copper foil is masked with use of a resist, such as a photoresist, printing resist, etc., and undesired portions of the foil surface except conductor circuits are removed by etching.
According to this method, however, it is economically difficult to reduce the thickness of the copper foil of the copper-clad substrate on account of problems on processes of production. Essentially, therefore, this method is unfit for the production of thin-film versions of conductor circuit boards. It is necessary, moreover, to use a copper foil with a thickness of 18 .mu.m or more in order to fully stand a tensile force, bending force, etc., which are physically applied during processes after the production of the copper foil, including surface treatment, cutting, and lamination on the insulating substrate. It is therefore difficult to obtain a copper-clad substrate with a copper-foil thickness of 5 to 10.mu. which are required for the production of higher-density versions of conductor circuits.
As means for settling these problems of the etching method, conductor circuit boards are conventionally known which are manufactured by the so-called transfer method. Examples of such conductor circuit boards are disclosed in Japanese Patent Publication No. 55-32239 (U.S. Pat. No. 4,053,370), Japanese Patent Publication No. 57-24080, Japanese Patent Publication No. 57-39318, Japanese Provisional Patent Publication No. 60-147192, etc.
In a method (hereinafter referred to as belt transfer method) of producing conductor circuit boards disclosed in Japanese Patent Publication No. 55-32239 (U.S. Pat. No. 4,053,370), Japanese Patent Publication No. 57-24080, and Japanese Patent Publication No. 57-39318, a conductor circuit board is produced in the following manner. A resist mask is applied to the surface of a thin, electrically conductive metal belt which slides on the outer peripheral surface of a metallic rotating drum or a cathode portion of a horizontal plating apparatus. The metal belt, for use as a cathode, is transported while being kept at a predetermined distance from an insoluble anode. A plating solution is supplied compulsorily between the metal belt and the anode at high speed, thereby electrolytically forming a conductor circuit on the surface of the metal belt. After an insulating substrate, having a bonding agent previously applied thereto, is adhered to the conductor circuit, the insulating substrate and the conductor circuit are peeled from the metal belt, and the conductor circuit is coated with an overlay for lamination, as required. Thus, the conductor circuit board is completed. Adapted for high-speed plating, the belt transfer method has the advantage over the conventional etching method that it permits very fast formation of the conductor circuit and continuous production of the conductor circuit boards. However, the belt transfer method is subject to the following drawbacks. During a peeling step in which the insulating substrate, having the conductor circuit transferred thereto, is peeled from the metal belt, part of the conductor circuit may not be able to be transferred to the insulating substrate, due to the difference between the strength of adhesion between the conductor circuit and the metal surface and that between the resist and the metal belt surface, and other causes. For the same reason, moreover, the material of the resist mask may be transferred to the insulating substrate, or the circuit may swing or be deformed during the transfer and peeling steps, thereby causing such defects as a short circuit, wrinkling, breakage, bruises, cracks, etc.
Using the metal belt as the electrically conductive substrate, moreover, the belt transfer method has a defect such that the metal belt, if having a substantial width, undulates as its travels, so that it is difficult to maintain a fixed distance between the metal belt and the anode. Therefore, the thickness of the conductor circuit, which is electrolyzed on the metal belt, varies according to location. If the conductor circuit has a fine circuit pattern, it is poor in dimensional stability during the transfer step and in yield. Thus, the belt transfer method does not permit the use of a wide metal belt, and can be improved only limitedly in productivity.
In the so-called reel-to-reel system disclosed in Japanese Patent Publication No. 57-24080, for example, a metal belt of stainless steel on a reel is wound therefrom by means of another reel. With this arrangement, the surface of the stainless-steel plate may suffer flaws, soiling, or other damages, and a resist pattern is liable to be soiled or flawed when it is applied. If work is suspended to remove soil or flaws on the pattern, the formation of the circuit, in its turn, is spoiled. Thus, according to the reel-to-reel system, the work (line) cannot readily be suspended even when the resist pattern suffers soiling, flaws, or other damages. This results in an increase in fraction defective, reduction in working efficiency, etc.
If stainless steel is used for the metal belt, moreover, inevitable physical or electrochemical defects, such as pores, exist on the surface of the metal belt. According to the belt transfer method, the conductor circuit is electrolytically precipitated in a direct manner on the surface of the metal belt with such defects, and is therefore liable to suffer pinholes. Such a situation is particularly important to high-density conductor circuit boards with a copper circuit width of 100 .mu.m or less and a circuit interval of 100 .mu.m or less.
Furthermore, FIG. 22 is a sectional view showing a step of overlay lamination based on the belt transfer method. A conductor circuit 92 bonded to an insulating substrate 91 by the belt transfer method projects above the surface of the substrate 91. Therefore, an overlay film 93 cannot be fully adhered to the outer peripheral surface of the conductor circuit 92 from over the same. Accordingly, air layers (voids) 94 are produced between the overlay film 93 and the conductor circuit 92. If the air layers 94 exist, the conductor circuit 92 is exposed to and oxidized by high-temperature air when it is heated in the overlay lamination step. It is difficult to entirely remove these air layers (voids) 94 by the prior art method, and this constitutes a primary factor in an increase of manufacturing cost. Further, the presence of the air layers 94 causes the conductor circuit 92 to be oxidized with the passage of time, as well as by the heating in the overlay lamination step. In the overlay lamination step, moreover, the insulating substrate 91, the conductor circuit 92, and the overlay film 93 are held between upper and lower rolls 95, 95, as shown in FIG. 22, when they are pressure-bonded with heat. According to the belt transfer method, however, the circuit 92 is adhered to the insulating substrate 91 so as to projects above the substrate 91, as mentioned before. As it is pressed by the rollers 95, therefore, the conductor circuit 92 is dislocated (swung) in the directions of the arrows of FIG. 22, with respect to the insulating substrate 91. Thus, the dimensional stability is poor.
A method (hereinafter referred to as conventional transfer method) of producing conductor circuit boards disclosed in Japanese Provisional Patent Publication No. 60-147192, mentioned before, comprises a step (FIG. 23(a)) of forming a thin metal layer on a metal substrate, a step (FIG. 23(b)) of roughening the surface of the thin metal layer, a step (FIG. 23(c)) of forming a plating resist on the whole surface of the thin metal layer except that portion thereof which is to be formed with a conductor circuit, a step (FIG. 23(d)) of forming the conductor circuit by plating the surface of the thin metal layer formed with the plating resist, a step (FIG. 23(e)) of peeling the thin metal layer, the conductor circuit, and the plating resist together from the substrate and transferring them to an insulating substrate, a step (FIG. 23(f)) of peeling off the metal substrate, and a step (FIG. 23(g)) of removing the transferred thin metal layer by etching. This conventional transfer method has the advantage over the aforementioned belt transfer method in that the conductor circuit can be transferred easily and securely in the following manner. A thin metal layer of about 1 to 10 .mu.m is previously formed on the substrate, and the resulting structure, along with the plating resist and the conductor circuit, is transferred to the insulating substrate. The surface of the thin metal layer is roughened by chemical etching, using a mixed solution of cupric chloride and hydrochloric acid. By doing this, good adhesion of the resist and the conductor circuit plating film to the thin metal layer can be maintained. However, the conventional transfer method indispensably requires the step of roughening the surface of the thin metal layer after forming the thin metal layer on the substrate in the aforesaid manner. This roughening process takes much time, thus exerting a bad influence on the improvement of productivity, and constituting a hindrance to the simplification of manufacturing processes.
In order to improve the adhesion between the conductor circuits and the insulating substrate, on the other hand, the surfaces of the conductor circuits must have a predetermined roughness.